<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>Title</title>

    <style>
        .menu-bar ul ul {
            display: none;
        }

        .menu-bar li:hover {
            cursor: pointer;
        }

        .menu-bar li:hover > ul {
            display: block;
        }
    </style>
  </head>
  <body>
    <!--1.鼠标移入时打开下拉列表-->
    <div class="menu-bar">
      <ul>
        <li>
          <div style="color: darkred">hover1</div>
          <ul>
            <li>
              <div style="color: red">no-hover2</div>
            </li>
            <li>
              <div style="color: red">hover2</div>
              <ul>
                <li>
                  <div style="color: plum">hover3</div>
                  <ul>
                    <li>
                      <div>End</div>
                    </li>
                    <li>
                      <div>End</div>
                    </li>
                    <li>
                      <div>End</div>
                    </li>
                    <li>
                      <div>End</div>
                    </li>
                  </ul>
                </li>
                <li>
                  <div style="color: plum">no-hover3</div>
                </li>
              </ul>
            </li>
          </ul>
        </li>

        <li>
          <div style="color: darkred">no-hover1</div>
        </li>
      </ul>
    </div>

  </body>
</html>